Corneal surgery is currently undergoing rapid evolution with improvements designed to minimize or eliminate astigmatism following penetrating keraplasty (corneal transplants), as well as to correct refractive error. Because the cornea is the most powerful refracting surface of the eye, numerous procedures have been devised to incise, lathe, freeze, burn and reset the cornea to alter its shape. Currently practiced keratorefractive surgical techniques include: cryorefractive techniques (keratomileusis, keratophakia, ipikeratophakia), radialkeratotomy, thermal keratoplasty, corneal relaxing incisions and wedge resections.
When preparing the patient for any of these surgical techniques, it is essential to accurately measure the corneal curvature. Existing methods to measure corneal curvature include central keratometry and photokeratoscopy with central keratometry. However, with these methods the diameter of the cornea that can be accurately measured is limited. Recently, photokeratoscopy has been adapted to provide a topographic map of the cornea. However, existing keratometers are limited in two important regards. Firstly, these instruments are predicated on geometrical image forming principles and assume the corneal topography can be expressed in terms of zones of various spherical radii. This in turn involves assumptions as to the nature of the surface under test. With more strongly aspheric corneas or as larger areas on even the average or typical size cornea are considered, the measurement becomes extremely ambiguous. Secondly, primarily as a consequence of the above but also because of optical engineering considerations, most instruments are limited in terms of the aperture of the cornea that can be measured. Existing instruments typically cover corneal diameters no greater than three millimeters. An additional problem one must consider in the design of a clinically useful instrument is the relatively low corneal specular reflectivity which is approximately two percent in the visible light range and virtually zero in the infrared light range.
While it is recognized that optical interferometric techniques for non-invasive measurement of corneal topography provide the only way of producing a contour map of the corneal surface directly, it is also recognized that visible light interferograms (operating wavelengths less than 0.7 microns) would typically result in the requirement to contend with hundreds of densely packed fringes even when the cornea under test is compared to an optimally fitting reference sphere. This problem is a consequence of the strongly aspheric nature of the typical cornea. Such interferograms require commensurately high resolution detection well beyond for example, the range of currently available detector arrays. Furthermore, such high submicron topographic sensitivity resulting from the use of such short wavelength light is unnecessary. If the reflectivity of the surface of the eye were not negligible in the infrared range (10-50 microns), a standard interferometric test performed using a single infrared wavelength would be an ideal solution, because no new technology would be required. Unfortunately, this is not the case. Thus there is still an ongoing need for a real-time keratometer system for medical diagnosis and for preparation of a corneal contour for eye surgery, as well as, for post-operative analysis of completed eye surgery.